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Antenna article
In article ,
Sal salmonella@food poisoning.org wrote: How would you change the feed method? I've had generally good performance from my J-poles but I'll gladly improve what I do, if you have some ideas. One of the older ARRL guides or antenna books shows a balanced method of feeding a J-pole. A standard half-wave coaxial balun is used. The two balanced outputs of the balun are tapped onto the two sides of the J-pole matching section, some distance above the usual "50-ohms-or- thereabouts" attachment point. My understanding is that the impedances "seen" on the two sides of the matching section won't be identical; the short side ends at an open-circuit point and the other side "ends" at the beginning of the half-wave section, where the impedance is high but not quite an open circuit. Hence, you won't achieve complete balance this way - there will probably be some current flow on the outside of the halfwave coax balun section. |
#2
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Antenna article
"David Platt" wrote in message ... In article , Sal salmonella@food poisoning.org wrote: How would you change the feed method? I've had generally good performance from my J-poles but I'll gladly improve what I do, if you have some ideas. One of the older ARRL guides or antenna books shows a balanced method of feeding a J-pole. A standard half-wave coaxial balun is used. The two balanced outputs of the balun are tapped onto the two sides of the J-pole matching section, some distance above the usual "50-ohms-or- thereabouts" attachment point. My understanding is that the impedances "seen" on the two sides of the matching section won't be identical; the short side ends at an open-circuit point and the other side "ends" at the beginning of the half-wave section, where the impedance is high but not quite an open circuit. Hence, you won't achieve complete balance this way - there will probably be some current flow on the outside of the halfwave coax balun section. I don't know which one would be the best but I have seen 3 methods of feeding the J-pole. If you insulate the bottom then you hook the feedline to the bottom with the center of the coax to the long side. If you do not insulate the bottom you tap up the matching segment so that you get a 50 ohm (if that is the coax used) match with the center of the coax connected to the long leg. Then there is the balun made out of coax that is hooked up to the matching segment so that a low swr is obtained. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com |
#3
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Antenna article
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#4
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Antenna article
In article ,
W5DXP wrote: Don't forget the Arrow Open Stub J-pole. http://www.arrowantennas.com/osj/j-pole.html There is (or was) a nice writeup of this variety on Cebik's web site. He refers to it as a "variant J-pole". The feed arrangement is different (it's open at the feedpoint, rather than being fed a few inches above a short), and the arm lengths are different than with a "classic" J-pole. The current distributions are different, of course, but the radiation pattern is only very slightly different than the "classic" variety. I have one of these dual-band types in my "go-kit" (it's actually a knock-off, built locally based on the plans that Arrow used to have on their web site) and it's served me well. I usually mount it at the top of a two-or-three-segment aluminum mast, sitting in an old hefty movie-camera tripod base. Much too large and heavy for a vehicle, of course, but I imagine you could build a somewhat-similar open-stub J-pole using much lighter materials (e.g. fiberglass whips with wire fastened inside or outside). |
#5
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Antenna article
On Tuesday, February 18, 2014 6:26:40 PM UTC-6, David Platt wrote:
The feed arrangement is different (it's open at the feedpoint, rather than being fed a few inches above a short), and the arm lengths are different than with a "classic" J-pole. If one will ignore the long 2m element and draw a schematic of it just for the 70cm band, one will realize that it is actually just a standard Zepp antenna. |
#7
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Antenna article
On Tue, 18 Feb 2014 18:10:39 -0500, "Ralph Mowery"
wrote: I don't know which one would be the best but I have seen 3 methods of feeding the J-pole. If you insulate the bottom then you hook the feedline to the bottom with the center of the coax to the long side. If you do not insulate the bottom you tap up the matching segment so that you get a 50 ohm (if that is the coax used) match with the center of the coax connected to the long leg. Then there is the balun made out of coax that is hooked up to the matching segment so that a low swr is obtained. There's also the American Legion J-Pole or the Silicon Valley Emergency Communications Systems J-Pole: https://picasaweb.google.com/112916124640757906440/NonarthopodicAntenna#5459396072666399154 https://picasaweb.google.com/112916124640757906440/NonarthopodicAntenna#5459396111364421106 This design does one thing right that none of the other J-Pole mutations seem to consider. The length of the wire between the coax connector center conductor and the driven element is an inductor. In order to tune out this inductance, one needs a series capacitor, with the inductor and capacitor tuned to the operating frequency. In other words, a gamma match. The series capacitor is formed by the insulated turns of electrical wire wrapped around the driven element. Another thing this design does right is use the zero current point at the bottom of the antenna as a ground. The problem is that it also extends the length of the center wire, which makes using a gamma match all the more important. I think putting the 50 ohm feed point and the corresponding ground close to each other were either to reduce the inductance of the connecting wire, or some manner of mutation from when it was fed by a balance line. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#8
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Antenna article
"Jeff Liebermann" wrote in message ... ... The length of the wire between the coax connector center conductor and the driven element is an inductor. In order to tune out this inductance, one needs a series capacitor, with the inductor and capacitor tuned to the operating frequency. Yes. I've been making copper pipe and 2-wire transmission line J-poles for almost 20 years. With pipe, I usually fasten the two feed points with clamps and slide the connections up and down. I'll get a VSWR low-point in-band but early-on, I discovered that the best VSWR was often about 1.7:1. I had read about (but never built) a gamma match, so I'd heard about the series cap to tune out the inductance. I tried a series cap at the feed and it helped. 70 - 100 pF seems to be about right at 2m and I can often get a 1:1 reading somewhere in the band. Does such a 0.3 dB improvement matter? That's not my call. When I'm essentially playing with the technology, I can take more time than if I'm working, like to a deadline or a budget. "Sal" |
#9
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Antenna article
On Wed, 19 Feb 2014 21:16:19 -0800, "Sal" salmonella@food
poisoning.org wrote: "Jeff Liebermann" wrote in message .. . ... The length of the wire between the coax connector center conductor and the driven element is an inductor. In order to tune out this inductance, one needs a series capacitor, with the inductor and capacitor tuned to the operating frequency. Yes. I've been making copper pipe and 2-wire transmission line J-poles for almost 20 years. With pipe, I usually fasten the two feed points with clamps and slide the connections up and down. I'll get a VSWR low-point in-band but early-on, I discovered that the best VSWR was often about 1.7:1. I had read about (but never built) a gamma match, so I'd heard about the series cap to tune out the inductance. I tried a series cap at the feed and it helped. 70 - 100 pF seems to be about right at 2m and I can often get a 1:1 reading somewhere in the band. I'm not sure of the frequency of your J-pole. The inductance of about 6 cm of #12AWG solid wire is about 0.05 uH. http://www.consultrsr.com/resources/eis/induct5.htm To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be about 200 pf. Does such a 0.3 dB improvement matter? That's not my call. When I'm essentially playing with the technology, I can take more time than if I'm working, like to a deadline or a budget. 0.3dB is about 6.7% loss. Probably not important or toss a coin? -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#10
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Antenna article
"Jeff Liebermann" wrote in message ... I'm not sure of the frequency of your J-pole. The inductance of about 6 cm of #12AWG solid wire is about 0.05 uH. http://www.consultrsr.com/resources/eis/induct5.htm To resonate at 146Mhz, that would be about 24 pF. 50 Mhz would be about 200 pf. That's enlightening. I need to admit that no such science was applied. I just tried small caps from my stash to see what would happen. When something good happened, I remembered it for next time. The j-pole that seems to benefit most is the 2m version, which I've built the most of. Perhaps we're not dealing with just the inductance of that 6 cm of wire. There could be some residual inductance in the rest of the antenna which is being brought to resonance. Maybe when my element lengths are not optimal, some inductance would be found there. Not sure why the residual reactance would always be inductive. The first time I add a cap and it gets worse, then I'll know it isn't so. Maybe I should experiment with varying the spacing across the stub and see what that does, with and without a cap. "Sal" |
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